1. Field of the Invention
The present invention relates generally to an automatic frequency control (AFC) circuit and more particularly to an AFC circuit, preferably is used in a television receiver, which has a predeterminedly limited control range.
2. Description of the Prior Art
In a prior art horizontal AFC circuit of a television receiver, as shown in FIG. 1, the output from a horizontal oscillator 1 which can be, for example, a control pulse is supplied to a horizontal output circuit 2 and a flyback pulse therefrom is fed to a saw-tooth wave generator 3. Though not shown, the horizontal output circuit 2 comprises a deflection circuit and a high voltage generating circuit. The saw-tooth wave signal from saw-tooth wave generator 3 is supplied, as a comparison signal, to a phase comparator 4 which is also supplied with a horizontal synchronizing (sync) signal H through a terminal 5. In phase comparator 4 the saw-tooth wave signal is compared with the horizontal sync signal H to detect the phase difference between the saw-tooth wave signal and the horizontal sync signal. The detected phase difference is applied to horizontal oscillator 1 as an AFC voltage e.sub.c. As shown in FIG. 2, when the horizontal sync signal H is coincident with the center .phi..sub.0 of the downward sloping portions of the comparison signal, that is, when the phase difference between the comparison and sync signals is 0 (zero), the synchronization is maintained for phase differences of which corresponds to the ends of the downward sloping portion of the comparison signal. For phase differences of .+-..phi..sub.cm, the AFC output voltage e.sub.c has a maximum value of .+-.e.sub.cm.
If the AFC control sensitivity is taken as .beta., the maximum frequency range within which the f.sub.cm oscillating frequencies are controlled, hereinafter referred to as the control range, is: EQU f.sub.cm =.+-.2.pi..beta.e.sub.cm
When the value of e.sub.cm in the above formula is need constant, regardless of frequency variation, then the value of f.sub.cm is constant.
Generally, the comparison signal, which is supplied by saw-tooth wave generator 3, is provided by integrating the flyback pulse. Both the width of the flyback pulse, which is determined by an LC resonance of horizontal output circuit, and the inclination of the rising portion of the comparison signal, which is determined by an RC time constant of the sawtooth generator, are constant. Therefore, the value of e.sub.cm varies with the frequency resulting in a variable value of f.sub.cm.
For example, as shown throughout FIGS. 3A, 3B, and 3C, both the width of the downward sloping portion and the inclination of the rising portion of the comparison signals are constant. As a result, when the frequency becomes high (FIG. 3A) as compared with its reference state (FIG. 3B), the value of e.sub.cm becomes small as represented by e'.sub.cm and when the frequency becomes low (FIG. 3C) as compared with the reference state (FIG. 3B), the value of e.sub.cm becomes large as represented by e".sub.cm.
Therefore, when the frequency of the horizontal output signal is high, the differences between the minimum and maximum control signal amplitudes, hereinafter referred to as the amplitude range become smaller resulting in a reduced control range f.sub.cm. In contrast thereto, when the frequency of the horizontal oscillator signal is low, the amplitude range becomes larger resulting in an increase of control range f.sub.cm.
Typically for a variable control range as described heretofore, the AFC circuit is designed with the smaller control range, corresponding to high oscillating frequencies, as a reference. Such a reference, however, results in the control range at lower oscillating frequencies becoming either too large or at least larger than necessary.
When the control range at the lower oscillating frequencies is too large, the amplitudes of the control signals, that is, the values of .+-.e.sub.cm, are too high or low to be applied to horizontal oscillator 1 and thereby results in an unacceptable frequency correction. Such unacceptably high or low values of e.sub.cm can occur, for example, when the sync signal disappears during switching of television channels resulting in the oscillating frequency becoming too low and thereby creating a voltage in the horizontal output circuit that is abnormally high. Therefore, it is necessary that the control range of the AFC circuit be made as small as possible for lower frequencies.
Further, if the control range is too large, the AFC circuitry may unnecessarily adjust the oscillating frequency when noise is present on a weakly received signal.
Accordingly, an AFC circuit should necessarily have as small a control range as possible. Such a small control range is possible by providing maximum and minimum values of e.sub.cm irrespective of frequency variation.